The present invention generally relates to voltage controlled oscillator circuits, and more particularly to a voltage controlled oscillator circuit which is used for example when forming an internal clock signal generating circuit provided in a digital signal processor (DSP), a microprocessor unit (MPU) or the like.
FIG. 1 shows an example of a conventional voltage controlled oscillator circuit. This voltage controlled oscillator circuit is formed by a GaAs integrated circuit and includes an oscillator circuit part 1 and an output circuit part 2 for obtaining an oscillation output of the oscillator circuit part 1. In FIG. 1, V.sub.DD denotes a power supply line.
The oscillator circuit part 1 includes a voltage controlled current source 3 and a ring oscillator circuit 6.
The voltage controlled current source 3 includes a control voltage input terminal 4 and a depletion type metal semiconductor field effect transistor (MESFET) 5. An output current I.sub.0 of the voltage controlled current source 3 is controlled by a control voltage V.sub.C which is applied to the control voltage input terminal 4.
The ring oscillator circuit 6 includes inverters 8, 9 and 10. More particularly, the ring oscillator circuit 6 includes resistors 11, 12 and 13, enhancement type MESFETs 14, 15 and 16, and capacitors 17, 18 and 19. The resistors 11 through 13 form load elements, the enhancement type MESFETs 14 through 16 form driving elements, and the capacitors 17 through 19 form delay elements.
On the other hand, the output circuit part 2 includes a level conversion circuit 20 and an inverter 25.
The level conversion circuit 20 converts the level of an oscillation output received from the ring oscillator circuit 6. The level conversion circuit 20 includes Schottky diodes 21, 22 and 23, and a depletion type MESFET 24.
The inverter 25 includes a depletion type MESFET 27 and an enhancement type MESFET 28. The depletion type MESFET 27 forms a load element, and the enhancement type MESFET 28 forms a driving element.
FIG. 2 shows an operation characteristic of the voltage controlled oscillator circuit shown in FIG. 1. In FIG. 2, a solid line 29 indicates a change of the oscillation frequency of the ring oscillator circuit 6 with respect to the control voltage V.sub.C, and a solid line 30 indicates a change of a center level of the oscillation output of the ring oscillator circuit with respect to the control voltage V.sub.C.
According to the voltage controlled oscillator circuit shown in FIG. 1, since the voltage controlled current source 3 which controls the oscillation frequency of the ring oscillator circuit 6 is formed solely by the single depletion type MESFET 5, a current will flow between the gate and source of the MESFET 5 if the control voltage V.sub.C is increased to a voltage of -0.4 to 0.8 V. As a result, it was impossible to greatly change the control voltage V.sub.C, and there was a problem in that the oscillation frequency range cannot be made large.
On the other hand, the level conversion circuit 20 of the output circuit part 2 uses the Schottky diodes 21 through 23. For this reason, the Schottky diodes 21 through 23 may turn OFF when the center level of the oscillation output of the ring oscillator circuit 6 varies, and there was a problem in that the oscillation output may not be obtained in such a case.